Butadiene separation



Jan. 28, 1947.

K. H. HAACHMUTH BUTADINE SEPARATION Filed. Aug. 10, 1942 HOLVNOILDVtIdlN'vl-:NToR KARL- H. HACHMUTH ATTO Patented .,Jan. 28,

BUTAIENE SEPARATION Karl H. Hachmuth, Bartlesville, Okla., assignor toPhillips Petroleum Company, a corporation of Delaware Application August10, 1942, Serial No. 454,312

This invention relates to the recovery of butadiene-1,3 from a complexpetroleum mixture comprising bo'th normally gaseous and normallyliquidhydrocarbons of both saturated and unsaturated linkages. Morespecifically, it relates to a process for recovering substantially purebutadiene- 1,3 from a petroleum fraction consisting principallyofpropane, propylene and lighter hydrocarbons, isobutane, isobutylene,butene-l, butadiene-1,3, n-butane, butene-Z (both high and low boiling),and a heavy fraction of and more carbon atoms containing a largepercentage of diolens. The invention further relates to the recovery ofbutadiene-1,3 from a complex petroleum fraction containing, in additionto such hydrocarbons as described above, small quantities of.

both methylacetylene and vinylacetylene.

One source of butadiene-1,3 is from various hydrocarbon cracking andreforming processes in which the butadiene may be recovered as abyproduct from the'reaction effluents. The economical recovery ofsubstantially pure butadiene from the other reaction products presents adifficult separation problem, for a number of reasons among which arethe following:

1. Butadiene is usually present in small concentrations in the reactioneffluents, thereby requiring the handling of large volumes of material.

2. Many four carbon atom hydrocarbons present with the butadiene in thereaction mixture have boiling points very close to that of butadiene.For this reason, and also because of the fact that the concentration ofbutadiene in the reaction mixture is usually very small, extremelylcareful fractionation must be practiced to obtain the maximum recoveryof high purity-butadiene.

3. An azeotrope is formed between butadiene and n-butane. Some meansother than conventional fractionation must therefore be employed toremove the butane from the butadiene if the butadiene recovery andpurity are` to be kept at amaximum.

4. The C5 and heavier fraction usually contains a large percentage ofdiolens, for example, cyclopentadiene, isoprene, and piperylene, whichundergo non-catalytic polymerization at or above room temperature. It isadvantageous, and usu ally essential, for reasons disclosed below toremove these materials early in the separation process.

5. Small amounts of acetylenes, principally methylacetylene andvinylacetylene, may be pres-l ent in the C4 fraction. Since anyappreciablev quantity of these compounds is inJurious to the 4Clalms.(Cl. 202-39) quality of synthetic rubber manufactured from thebutadiene. the separation steps used should include a provision fortheir removal.

Therefore, adwith'the above considerationsin mind, the principal objectof this invention is to provide an economical and practical arrangementof separation steps for the recovery of butadiene-1,3- from a complexhydrocarbon mixture such as previously described. Numerous other objectswill more fully appear hereinafter.

The accompanying drawing portrays diagrammatically one arrangement ofequipment which has been found useful in carrying out thepresentinvention in one embodiment thereof.

My invention will first be described in connection with the preferredembodiment wherein a butadiene-containing stream which also contains Cahydrocarbons including methylacetylene,

'C4 hydrocarbons boiling close to butadiene 1nd including butene-Z,normal butane,fvinylacetylene, possibly isobutane although it may beabsent, generally butene-l, generallyisobutylene, usually C5hydrocarbons, and frequently heavier hydrocarbons than C5.

The butadiene-containing feed is first subjected to a depropanizingstep. This step has for its particular purpose, in addition to removalof propane and propylene, the removal of the methylacetylene which wouldotherwise appear as a contaminant in the butadiene. Consequently,this-step is necessay whether or not the volume 0f Cs hvdrocarbons inthe initial feed stream is large or small. If the volume of Cahydrocarbons in the initial feed stream is large, this step, of course,has the further advantage in that it reduces the load on subsequentIequipment.

The extent of the removal of methylacetylene is dependent upon theextent or degree of depropanization; that is, the more complete thedepropanization of the feed the more complete the removal of themethylacetylene. For example, the following tabulation gives anapproximate indication of the relation between the extent ofdepropanization and the extent of removal of methylacetylene by afractionator having a moderate number of trays: g

Per cent propane Per cent methyl-` remov acetylene removal It can beseen from the above tabulation that essentially complete depropanizationof the feed is highly desirable to reduce the methylacetylene content toa low value. Also, it can be seen that with essentially complete propaneremoval, roughly 20 to 30 per cent of the methylacetylene originallypresent will remain in the feed. However, the amount of this componentin the orisinal feed stream to the depropanizing step is usually lessthan one per cent. Consequently, when '10 to 80 per cent of this one percent of methylacetylene is removed in the depropanizing step only asmall fraction remains. This small amount that remains in the bottomemuent from the depropanizing step is later removed in the absorptionstep.

The bottom fraction from the depropanizing step Iis subjected to afurther fractionation which is essentially for the purpose of removingthe Cs and heavier hydrocarbons and a large portion of the butenes-2.Not all of the butenes-2 can be economically removed by fractionationbecause .of the closeness of their boiling points (cls and trans) tothose of n-butane, butadiene, butane-1, and isobutylene, Consequently,the fractionation is carried out to remove as much of the butane-2 aspossible without loss of butadiene in order to reduce the load in thesubsequent absorption step elimination of normal butane along withbutene-l with furfural. The final removal o1' butene-2 is effected by afractionation step after the furfural absorption andstripplng steps.

As pointed out, the removal of as much of the butenes-2 as possiblebefore the absorption step is desirable from the standpoint of reducingthe load on the absorption equipment. However, since removal of all ofthe butenes-2 cannot be effected economically in the secondfractionation step, and since a third fractionation step has to beemployed after the absorbing stripping step to remove the residualbutenes-2, etc., it is frequently desirable to balance the load ofbutenes-2 removal between these two fractionation steps rather thanoperating the second fractionation step before the absorption-strippingsteps to remove as much of the butenes-2 as possible. Also. the secondfractionation step removes someof the vinylacetylene- 'Ihe residualvinylacetylene is removed in the final distillation step after thefurfural absorption and stripping steps along with butene-Z. A

The overhead from the second fractionation step is subjected to afurfural absorption treatment which selectively absorbs butadiene andvinylacetylene to the essential exclusion 'of the other hydrocarbons,except butenes-Z, in the stream. The butenes-Z are distributed betweenthe overhead and bottom product. The primary purpose of this step is toseparate the isobutylene Y and butene-l from the butadiene. However,this step also results in an additional advantage in that removal ofnormal butane which forms an azeotrope with butadiene is simultaneouslyeffected. The reason for employing this step to remove Y butene-l andisobutylene is that it is difllcult to and isobutylene which wouldotherwise have to be removed by some other means.

It is also to be noted that normal butane cannot be removed frombutadiene by simple fractional distillation because of the azeotropeformed by these two compounds. Usually, however, normal butanerepresents only a comparatively small percentage of abutadiene-containing mixed hydrocarbon stream, and since the azeotropeis the minimum boiling type and, therefore, boils at a lower temperaturethan butadiene, it would appear that by taking slightly more butadieneoverhead than required to form the azeotrope with the normal butanepresent, the normal butane could be eliminated from the butadieneretained in the bottom product. However, in practice the separationcannot be effected in this manner which probably means that the othercomponents in the hydrocarbon mixture tend to partially break theazeotrope. In the previously discussed 200 plate fractionator with' areflux ratio oi 150 to 1. the indications were that if about 25 per centof the butadiene-1,3 was taken overhead with the butene-I andisobutylene, approximately only 50 per cent of the normal butane wastaken overhead. When using a furfural absorption system in accordancewith my invention, the normal butane can be removed overhead along withthe butene-l and isobutylenepractically 10.0 per cent with less than oneper cent loss of butadiene. The combined amount of-butene-l andisobutylene retained in the furfural along with butadiene is usuallyless than one per cent.

. As pointed out above, the overhead products from the secondfractionation step are, in accordance with my invention, subjected to afurfural absorption treatment wherein the butadiene, vinylacetylene, andmost of the butene-2 are selectively absorbed While butane-1,isobutylene, butanes, and a small portion of the butene-Z are not.However, to effect this separation it is essential that a certain amountof reboil heat be applied to the furfural absorber. Furfural isselective to varying degrees towards all of the different hydrocarbons.Those towards which it is less selective are easily stripped from itwhile those towards which it is most selective are more diflicultlyremoved. ,The reboil heat which is applied at the bottom of, thefurfural absorber functions to strip out the less selectively absorbedcompounds consisting, in the usual case, primarily of butene-l',isobutylene, and normal butane. The butadiene, vinylacetylene, and mostof the butene-Z are retained in the furfural. Of course, to removenormal butane, isobutylene, and butene-l essentially completely, it isnecessary to strip out some butadiene in the bottom of the absorber.However, the butadiene and undesirable components after being releasedpass upwardly through the furfural vtowards the top where the furfuralis cooler than at the reboiler, and the butadiene is again selectivelyabsorbed while the undesirable lessv selectively absorbe components,after the upper layers of furfural once become essentially saturatedwith them, are discharged overhead. y

The rich furfural from the absorption step is "separated from theabsorbed hydrocarbons in a stripping step. The resulting lean furfuralis returned to the absorption' zone. The overhead products from thestripping zone are subjected to a fractional distillation to removebutene2 and vinylacetylene from the butadiene to give essentially purebutadiene. The pure butadiene is recovered as an overhead product. Partof the purpose of this final fractionation step has already been pointedout in the discussion of the second fractionation step. It also servesto remove any furfural carried over from the stripping step and anypolymer of butadiene or furfural that might be present.

Butadiene' of very high purity can be obtained by my process. In plantoperations. butadiene of better than 99.5 per cent purity has beenobtained, namely 99.8, with about 98.5 to 99 per cent being about theaverage purity obtained consistently. In comparison with this, the datapreviously presented for the recovery of butadiene from butylenes,butanes, etc., by fractionation alone indicates that butadiene of highpurity can be Obtained provided normal butane is not present or itsconcentration in the feed stock is very W. but even so, considerablebutadiene necessarily has to be wasted to effect essentially completeremoval of the butylenes. However, normal butane, as wellas isobutyleneand butene-l, is usually present in butadiene-containing feed streamsand when present inquantities equal to or less than that required toform the azeotrope with butadiene, it tends to become the limitingfactor in regard to the purity of the butadiene that can be recovered byfractionation alone, bec'ause in fractionation it goes overhead with thebutadiene. In concentrations higher than that required to form theazeotrope, the purity of the butadiene recovered will be approximatelyequal to its concentration in the azeotrope, that is, about 80 per cent.

In accordance with my invention, a predominantly C4 stream comprisingbutadiene and butene2, with or-without other hydrocarbons, may besubjected to selective solvent extraction with asolvent for thebutadiene under conditions such that only a portion of the butene2 isdissolved. The rich solvent is then stripped in the usual :vay drivingoff the dissolved butadiene and butene2.

, Thereupon, and this is a most important feature of this embodiment ofmy invention, the recovered mixture is fractionally distilled toseparate yessentially pure butadiene, 'i. e. -at least 98% pure, on theone hand from the butene2 on the other hand.

In most cases the butadiene stream will contain normal butane. which hasheretofore involved difficulties of separation from butadiene. Such afeed stream may be treated in 'accordance with my invention to readilyrecover the butadiene in a high state of purity, at least 98%, by firstextracting with a. solvent selective for solvent extraction step isprimarily or exclusively a C4 stream. In addition to the speciilc C4hydrocarbons referred to above, namely, butadiene,

'toseparate from the other components present by using conventionalmethods, such as fractional distillation, because of the closeness ofits boiling point to that of some of the other components of themixture, because it possibly forms azeotropes with other components ofthe mixture, and possibly rbecause of its tendency to decompose orpolymerize.

When proceeding in accordance with my 4invention, however, anyvinylacetylene present inthe C4 stream being treated is dissolved, alongwith the butadiene, in the extraction solvent. During the fractionationof the mixture recovered by stripping the rich solvent the butadiene isreadily separated overhead while the vinylacetylene is removed in thebottoms along with butene2 if present.

The butadiene-containing C4 stream may advantageously be prepared fromany available stream containing hydrocarbons lighter and/or heavier thanC4, in accordance with a more speciilc aspect of my invention. Forexample, such a stream, which may have been produced by cracking,dehydrogenation, etc., may be flrst fractionated in a depropanizing stepto separate C3, methylacetylene and lighter hydrocarbons; whereupon thebottoms may be fractionated in a debutanizing operation to recoveroverhead all the C4 constituents originally present except a part of thebutene2., This C4 overhead product may then, either with or withoutinterposition of still another fractionation to separate any isobutanepresent, be subjected to solvent extraction and the succeeding stepsdescribed above forI the recovery of the pure butadiene.

Removal of any isobutane in the manner just alluded to further lowersthe volume load on the solvent extraction step.

Frequently the original stream, except for a stream obtained bydehydrogenating C4 hydrocarbons, vbefore separation of the C4 stream bydepropanizing and debutanizing, will preponderantly comprise Cs andlighter. Often these will amountl to or more of the original stream.However, for a stream obtained by dehydrogena-` tion of C4 hydrocarbonsthe depropanizing step is still necessary for the removal ofmethylacetylene.

The original stream may contain appreciable amounts of methylacetylene.This is essentially completely removed in the first` fractionation step.Also the Cs content of the stream may comprise appreciable amounts of Csconjugated di butadiene whereby all the butadiene and only 4a portion ofthe butene2 in the feed are dissolved butnone or only a trace of thenormal butane is dissolved. Upon stripping and fractionating asdescribed in the preceding paragraph, pure butadiene is recovered asbefore.

pentadiene. These go out as bottoms in the debutanizing step,.andlikewise are prevented from interfering with the operation of theprocess.

Isobutane, it present, may or may not have been removed from the C4stream subjected to the solvent extraction. This may have been ef- 10fected in the depropanizing step by appropriate adjustment of the cut,or in an `intermediate fractionation step as referred to above.Isobutane is the lowest boiling of the C4 hydrocarbons apt to beencountered and therefore readily re'- It will be seen that the streamtreated by the Il moved. For reference purposes the boiling points 1 ofthe C: and C4 hydrocarbon apt to be present in the original stream aretabulated as follows:

As the selective solvent, furfural is by far the most highly preferred.However, other solvents having similar selectivity for butadiene may beused in place of furfurai, though less desirable. Examples are:nitrobenzene, dichloridiethyl ether, dimethyl formamide, methyllevulinate, glycol diacetate, diethyl acetamide, pyridine, phenol,phenol plus water, benzyl alcohol, etc.

The furfural used may be anhydrous but preferably contains up to 5% ofwater, usually 4 to 5%, in order to increase its selectivity forbutadiene, lower its boiling point, 'and bring about other advantages.Use of furfural containing significant amounts of water is disclosed andclaimed' in my copending applications, Serial No. 430,307, filedFebruary l0, 1942 and Serial No.

438.844,-1'lled April 13, 1942. Since the water is lost from the `systemby going overhead with the hydrocarbons evolved in the stripping column,it is convenient to add water lto the system from time to time orcontinuously. For example, this water may be introduced to the strippingcolumn, say at the top thereof, or to the lean furfural stream fed intothe top of the extraction column. It may be desirable to maintain thefurfural lin the system and especially in the absorber completely orsubstantially saturated with water at the temperatures involved.

The furfural or the like breaks any azeotrope of normal butane andbutadiene or prevents the formation of such azeotrope by dissolving thebutadiene. In fact the invention is applicable to an azeotrope ofbutadiene and normal butane. For example, such an azeotrope, whichcontains approximately 80% butadiene and 20% normal butane, and formedin any suitable manner, may be solventl extracted as described abovepreferably with furfural to recover pure butadiene.

The final fractionation to separate butadiene of at least 98.5% purityfrom butane-2 may ad-- vantageously be conducted in a columnofpreferably not less than 100 trays. The ilnal fractionation separates,out in the bottoms product any iurfural or other selective solventcarried in the overhead from the stripping column.

Referring to the drawing, from a source of supply, not shown, apetroleum fraction consisting principally of propane, propylene andlighter hydrocarbons, isobutanerisobutylene, butene-l, butadiene-1,3,normal` butane, butene-2 (both high and low boiling), and a C5 andheavier fraction containing a large percentage of dioleilns is chargedto fractionatcr 2 through line I. The mixture may or may not containdetect- -ablequantities of all of the above hydrocarbons and also may or.may not contain detectable quantities of acetylenes. Substantially allthe propane, propylene and lighter hydrocarbons. which will ordinarilyrepresent the greatest p0rtion of the feed to fractionator 2, as well asmost of any methyl-acetylene that may be present in the feed, areremoved from the top of fractionator 2 through line 3. Substantially allof the C4 and heavier hydrocarbons, along with any vinylacetylene thatmay be present in the feed, are removed from the bottom of thefractionator through line 4 and charged to fractionator 5.

In a butadiene by-product recovery plant wherein its feed consistsessentially of the reaction products from a petroleum cracking and/orreforming operation, large quantities of hydrocarbons boiling below C4'smay ordinarily be expected to be present in the charge stock to therecovery plant. Therefore, early depropanization has the advantage oflowering the volume of material to be handled throughout the rest of theseparation steps thereby reducing equipment size and investment cost.'Ihis step also has the further advantage of removing the major portionof any methylacetylene that may be present in the reaction mixture andwhich, if not removed, will be absorbed by the lean furfural (which isemployed subsequently as a selective solvent) in absorber 8; uponstripping of the rich furfural in stripper il, the methyl acetylene willpass overhead through line I2 to fractionator Il and will be removedoverhead from this fractionator through linev l5 thus appearing as animpurity in the nished butadiene product which is also withdrawn throughline I5.

Fractionator 5 is operated in such a manner as to remove substantiallyall of the Css and heavier components and part of the butene-2 from thebottom ofthe fractionator through line 1. The overhead product,consisting of butanes, butadiene-1,3, isobutylene, butene-l and butene-2 along with any vinylacetylene is removed via7 line 6 and charged toabsorber 8. Several advantages are gained by the fractionation stepconducted in fractionator -5. Among these advantages are;

1. Certain of the dioleflnic compounds that may be present in the Csandv heavier fraction, and especially conjugated diolefins, such asisoprene, piperylene and cyclopentadiene are quite valuable.. Suchdiolefinic hydrocarbons tend to polymerize at or. above roomtemperatures, the polymerization rate increasing rapidly withtemperature. By removing the Css early in the process the time ofprocessing them is reduced and they are not subjected to the hightemperatures of the absorbing and stripping operations, thus minimizingany loss of these valuable hydrocarbons through polymerizationreactions.

2. Over a period of time some of the olefinic compounds, particularlythe diolens, present in the C5 and heavier fraction polymerize to gummymaterials. Gum is not only undesirable because it deposits on heatingelements causing a reduction in heat transfer or because it may depositin lines, valves, etc., thereby reducing the ow,

but also because it will be absorbed in the furfural thus making itsregeneration more difficult. Thus -by early removal of the Cs andheavier, the extent of gum formation is markedly reduced.

3. Removal of the heavier material early in the process lowerssubsequent stripping and fractionation temperatures thereby reducing theloss of butadiene by polymerization.

4. A further advantage obtained by the use of fractionatorl 5 is thatthe load on the absorbing y bottom temperature of 175 F. Substantiallyall of the Cas and lighter hydrocarbons, consisting of about 'I0 partsof propane and propylene, 30

` parts of ethane and ethylene and a trace of ing coil 8A is employed inthe bottom ofabsorber I 8 to give a cleaner separation therein,inaccordance with principles now well-known to the art. yRich furiuralcontaining substantially all of the bute/diene charged to the absorberplus some butene-2 and any vinylacetylene that may be present iswithdrawn from the bottom of the absorber through line III and chargedto stripper II equipped with heating coil IIA. The product withdrawnfrom the top of absorber 8 contains A substantially all of theisobutylene, butane-1 and butanes in the feed to the absorber plus someof the butene-2 as well as any propane that may have been retained inthe product withdrawn from the bottom ci fractionator 2. y

In column II the rich furfural is wstripped of butadiene, butene-Z andvinylacetylene, if pres ent. These hydrocarbons pass overhead throughline I2 to fractionator I4. The lean furfural is withdrawn from thebottom of the stripper through line I3 and is recycled`to absorber 8.Since n-butanerv and butadiene form an azeotrope, pure butadiene can notbe obtained by l conventional fractionation. By my use of solventextraction to separate the butadiene from the n-butane at this stage inthe process, these difficulties are completely overcome in a simple andeconomical manner.

In the Ui'inal fractionation, butene-2 and vinylacetylene, if any, arecharged to fractionator I4 through line I2; substantially pure butadieneis removed overhead through line I5 and the butene-2 and anyvinylacetylene, plus any small amounts or furfural and polymer ofbutadiene or furfural that may have carriedfover from the strippercolumn I'Iare removed from the bottom of fractionator I4 through lineI6. Butadiene may be separated from Vadmixture with normal butane inazeotropic or any other proportions and with or' without otherhydrocarbons which may be close-boiling or not, by selective solventextraction with a solvent which' is selective for butadiene but whichdoes not appreciably dissolve normal butane.` Upon'strippingy of thebutadiene-rich solvent there is obtained butadiene free from normal.butaneand the formation of an azeotrope of the butadiene with thenormal butane, which occurs when efthe butadiene,

methane and ',methylacetylene were removed overhead through line 3.rSubstantially all of the C4 and heavier hydrocarbons, consistingofabout 32 parts butadiene, 7, parts butene-l, 3

parts isobutylene, 12 parts butene-2 (both high and low boiling), 2parts butane, and 44 parts of C5 and higher along with a small quantityof vinylacetylene were withdrawn from the bottom of fractionator 2through line 4 and charged to fractionator 5.- 4 I From f ractionator 5,which was operated at rI0 pounds per square inch absolute, reflux ratioof 25: 1, top temperature of 103 F. and bottom temperature of 220 F.,all of the .Cs and heavier material and about one-half ofthe butene-2was removed from the kettle. Y The overhead product, consisting i ofabout 3 parts propane, 63 parts butadiene, 13 parts butene-L 5 partsisobutylene,

y13 parts butene-2, and 3- parts of butane along with a trace ofvinylacetylene was charged to absorber 8.

v Lean furfural from stripper II was introduced into absorber 8 throughline I3. Rich furfural. containing principally about 4 parts ofbutadiene and 0.3 part of butene-2 (both high and low boiling) alongwith a trace of C5+ and vinylacetylerie Y was withdrawn from the bottomof absorber 8 forts are made to separate by fractional distillation amixture comprising these two ingredients, is effectively prevented.

Example As an example of the operation of my invention, an unsaturatedpetroleum fraction, from:

a source not shown(, containing about per cent through line I0 andcharged to stripper II.4 The product withdrawn from thetop of absorber 8through line 9 contained about 45 parts of butene-l, 10 parts ofpropane, 17 parts isobutylene,

'17 -parts of butene-2 (mostly low boiling) and 11 parts butane.- Theabsorber was operated at a pressure of 65 pounds per square inchabsolute, reux ratio of 16.611, top temperature of 120 F. and bottomtemperature of 240 F.

The-rich furfural was introduced into stripper II through line II). Thestripper was operated at apressure of v65 poundsper square inchabsolute, reflux ratio of 1:1, top temperature of 103 F. and bottomtemperature of 300F. Lean furfural containing 99.9 per cent furfural and0.1 per cent C4s was withdrawn from the bottom of the stripper throughlinel I3 and recycled to absorber 8.. The stripper overhead product,consisting of 93 parts butadiene and 7 parts of butene-2 along withtraces of isobutylene, butane-1, 05+, yvinylacetylene and furfural wascharged to fractionator I 4 through line I2.

In the nnal fractionator I4, operated at 65 pounds per square inchabsolute, reux ratio or 14:1, top and bottom temperatures of 103vand F.,respectively.'a finished butadiene product of about 98.5 per cent puritywas withdrawn overhead throughline I5. The bottoms product withdrawnfrom line' I6 consisted of about 97 parts butene-Z, 1 part butadiene,and 2 parts of Cs and heavier, furfural and vinylacetylene.

From the foregoing description a great man? advantages of. my inventionwill be apparent to those skilled in the art. Many of these have beenreferred to in detail above, Many of the quanti- 'tative advantages willnot be discussedin great 11 lowed by fractionation in the mannerdescribed in detail makes possible the clean separation of extremelypure butadiene. The invention enables the complete separation ofvinylacetylene from the butadiene which should be free fromvinylacetylene to be -acceptable as a starting material in syntheticrubber production. 'I'he invention also makes possible starting with acrude broad cracking, refinery; or dehydrogenation stream and enablesready elimination of the 'C3 and lighter including objectionablemethylacetylene and of the C and heavier including objectionable C5dioleiins together with a substantial portion of Ythe butene-2 which isfound in the bottoms from lthe second fractonator 5. Initial removal ofall these materials lightens the load on the solvent extraction unit andprevents interference with subsequent steps in. the process. Also thisremoval contributes materially to the high purity of the productobtained. Moreover the invention enables one to readily and economicallyobtain maximum yields-of butadiene even though the concentration in theC4 stream is low and though the original concentration in the ins itialcomposite feed is extremely low. lA great many more advantages will bereadily apparent to those skilled in the art.

By the terms fractionation and ufractionating" as used herein and in theclaims. I mean fractional distillation. As used herein butadiene meansbutadienel. The terms butene-2" and butenes-2" mean both the low andhigh boiling isomers.

1. A process forthe recovery of butadiene from a mixture of hydrocarbonscomprising butadiene and other closely-boiling C4 hydrocarbons includingbutene-l and butene-Z which comprises subjecting said mixture to a istfractional distillation keffecting removal of a portion of the butene-2,subjecting the remainder of said mixture to selective solvent extractionwith a solvent which is selective for butadiene under conditions suchthat a fraction consisting essentially of butadiene and a furtherportion of the butene-2 is absorbed without substantial absorption ofthe butene-l and the remainder of the butene-2, stripping said fractionof butadiene and butene-2 from the solvent, and subjecting said fractionto a second fractional distillation eiecting separation of the butadienefrom the butene-Z.

2. A process as dened in claim 1 wherein said solvent is f urfural.

3. A process for the recovery of butadiene from amixture of hydrocarbonscomprising butadiene and other closely-boiling C4 hydrocarbons includingnormal butane, butene-l, and vinylacetylene, which comprises subjectingsaid mixture to a rst fractional distillation effecting removal of aportion of the butene-2; subjecting the remainder of said mixture toselect ive solvent extraction with a solvent which is selective for.butadiene under conditions such that a fraction consisting essen`-tially of butadiene, vinylacetylene, and a further portion of thebutene-Z is adsorbed without substantial adsorption of the butene-l andthe remainder of the butene-Z; stripping said fraction of butadiene,vinylacetylene, and butene-2 from the solvent; and subjecting saidfraction to a second fractional distillation eiecting separation of thebutadiene from the vinylacetylene and butene-2.

4. A process as deiined in claim 3 solvent is furfural.

wherein said KARL H. HACHMUTH.

